Tinto Brass' Action. In the movie there's a porn-arty-movie crew that has to shoot a scene of an actress taking a dump (Paola Senatore, infamous for becoming a porn actress to sustain her drug addiction). Scene briefly visibile at the end of the trailer:

Friday the 13th part V: a victim-to-be is seen pooping in the can after complaining about enchiladas he ate. He gets p**sed when his girlfriend starts shaking the outhouse. She soon stops suddenly. He then goes outside and sees his girlfriend's throat is slit. He goes back in and the killer shoves a sword through the outhouse at the guy, finally killing him after several thrusts.

La Grande Bouffet. A group of middle-aged friends, bored with their humdrum lives, decide to eat like crazy till they die. One of them plays the piano and farts repeatedly until his intestines give out. His last moments are spent going out onto the patio, falling down and pooping out his guts.

Another one: 51st State/Formula 51. Sam Jackson and some other guy witness some punks poop themselves in some animal lab or something.Not Another Teen Movie: Sounds bad but this scene is cool: some dudes spy on a gal in the restroom from a ceiling duct. She has terrible gas and diarrhea. The floor eventually gives way and lands in a classroom during a teacher's lecture. Poop sprays from the falling toilet onto the teacher! Awesomely gross!